1. Field of the Invention
The present invention relates to methods of alleviating out-gassing problems occurring in vacuum-sealed devices and more particularly to a method of removing gases caused by out-gassing in a vacuum vessel by utilizing an appropriate time-and-temperature profile to activate a getter for use in the vacuum vessel containing temperature-sensitive materials.
2. Description of the Prior Art
In some evacuated devices, e.g., vacuum vessels, it is necessary to incorporate therein materials which give off gas with the passage of time. This gas reduces the quality of the vacuum, thereby making the device unsuitable for its intended use. This phenomenon is commonly referred to as "out-gassing."
One method for removing this gas as it occurs is to incorporate a device which will pump the gases and eliminate them from the vacuum system. An ion pump is suitable for this purpose; however, it requires a magnetic field and a high voltage source adding to the size, weight, complexity, and the cost of the device.
Another method of removing the gas is to include a getter material inside the evacuated device. A getter is a substance introduced into an evacuated device to remove harmful residual gases by chemical or physical action. The "gettering" properties of certain materials have been recognized by scientists since the late 1800's, and as the applications became more obvious, the principle of the getter has been employed.
One method of employing getters consists of using getter materials which are evaporable. In many vacuum devices, it is not always possible to employ an evaporable getter due to the lack of a suitable surface on which to deposit the evaporated material. Additionally, evaporated getters can migrate onto sensitive surfaces thereby preventing the device from functioning properly. Another method employs getter materials which require activation prior to use by raising the getter to high temperature in a vacuum for a specified period of time. The activation temperature can damage sensitive components within the evacuated device and its application has heretofore been very limited in temperature-sensitive devices.
One particular area for the application of getter technology is in the field of evacuated radiation-detector devices such as the SEMIRAD quartz-fiber dosimeter. The SEMIRAD quartz-fiber dosimeter is a fountain pen-sized radiation detector which contains a tissue equivalent plastic chamber operating in a vacuum on the SEMIRAD (Secondary Electron MIxed RAdiation Dosimeter) principle. Primary electrons resulting from gamma radiation and recoil protons resulting from neutron radiation cause low energy secondary electrons to be emitted from the walls of the plastic chamber. These secondary electrons are collected, causing the quartz-fiber electroscope to discharge, deflecting the image of the quartz fiber on a readout scale. Proper operation requires that the plastic chamber and electron collection volume be maintained at a vacuum of better than 1.times.10.sup.-4 Torr. Gases are given off from the plastic chamber and cause the vacuum to deteriorate with time. To help overcome this "out-gassing" problem, the dosimeter contains a small vacuum ion pump which can be activated by applying an appropriate magnetic field and a suitable high voltage. When good vacuum has been attained, the dosimeter may be returned to service. The complexity of the ion pump technique could be eliminated if an appropriate getter material could be incorporated inside the dosimeter without destroying or degrading performance characteristics of the dosimeter by by heat or vapor deposition.